The present invention relates to a three tube type color television camera especially for professional use, and in particular to a color separation system using prisms in the television camera.
A conventional three-tube type color television camera for professional use is illustrated in FIG. 1. Referring to this figure, light from an object (not shown) to be picked up is supplied to an optical system 11 for three-color separation through an image pickup lens 10. The optical system separates the received light into three primary colors of red (R), green (G) and blue (B). The separated three color components R, G and B are sent to image pickup tubes 12R, 12G and 12B, respectively. The image pickup tubes carry out the optical-electro conversion and produce video signals of corresponding colors. Those video signals are fed to corresponding pre-amplifiers 13R, 13G and 13B, which amplify the received signals. The output signals of the pre-amplifiers are then delivered to corresponding processing amplifiers 14R, 14G and 14B, which preprocess the output signals for subsequent color encoding. The output signals of those processing amplifiers are fed to a color encoder 15, which composes the received signals into a composite color video signal conforming, for example, to the NTSC (National Television System Committee) standard.
As the optical system 11 for the three color separation, an dichroic mirror system or an prism system using a three-color separation prism is known. Although the latter needs a particular image lens having a long lens mount to focal point distance, it is possible to design the optical system of small size, simple structure and high transmittance with the prism system. Therefore, most of the television cameras for professional use employ the prism system to which the present invention relates.
The description will now be given of a conventional optical system using prisms. FIG. 2 illustrates a conventional optical system of this type. Incident light from an object (not shown) passes through an image lens 10 (generally made up of a plurality of convex lenses), and then enters along an optical axis 20 into a blue separation prism 16B of a three-color separation prism system 16. The light reaches a dichroic mirror b.sub.1 provided on the inclined surface of the blue separation prism 16B, which is slanted with respect to the optical axis 20. There, only the blue component of the light is totally reflected on the dichroic mirror b.sub.1, leaving the other components to pass through into a red separation prism 16R of the three-color separation prism system 16 and then reach a dichroic mirror r.sub.1 provided on one of the inclined planes thereof. There, the red component is totally reflected on the dichroic mirror r.sub.1, leaving the remaining component, i.e. the green component, to pass through into a green separation prism 16G. Consequently, three primary color components are separated from the incident light passing through the image lens 10 and supplied to the corresponding image pickup tubes through corresponding trimming filters 17R, 17G and 17B. The trimming filters 17R, 17G and 17B mounted on the prisms 16R, 16G and 16B act to enhance the purity of the red, green and blue components, respectively.
In general, the three-tube type television camera as described above needs a series of adjustments prior to the start of use thereof. Those preadjustments include resolution, registration, centering and grey scale alignment procedures, for example. In detail, a uniformly illuminated test pattern, designed appropriate to a desired alignment procedure for the television camera, is located on the optical axis of the television camera so as to directly oppose the camera. The framing alignment is then carried out to obtain the specified viewing area of the image of the test pattern. Those alignment procedures are partly performed automatically by automatic adjustment circuits built into the television camera with assistance of the human operator. In such alignment procedures, the test pattern of a transparency or a positive illuminated by external light is generally employed. However, the alignment procedures are complex and cumbersome even with the automatic adjustment circuits, for instance, and the procedures require frequent exchange for different patterns. In addition, there is the possibility that the best pattern to be exchanged is not at hand but located remote from the human operator due to the lens system of the television camera.
In order to minimize the problem, a television camera with a built-in pattern projector was recently proposed. Such a television camera has been disclosed in the Japanese Patent Publication No. 7070/86, for example. This type of the proposed television camera with a built-in projector is based upon one of the following two arrangements.
(1) An arrangement in which the pattern image is introduced into the optical path of the camera, by using a light deflector which is interposed between two lenses forming an image pickup lens.
(2) An arrangement in which the pattern projector moves in front of the three-color separation prism system, replacing the bias light supply device which moves out when the alignment is carried out.
It should be noted that the arrangement (1) involves the construction of the image pickup lens making it difficult to be exchangeable. Otherwise, it is necessary to prepare one pattern projector for each image pickup lens. In other words, when the operator replaces every time his image pickup lens or zoom lens with another for picture-taking purposes, he must replace the pattern projector together for pre-adjustments. The construction of the arrangement (1) with exchangeable lenses is therefore complex and expensive. Further, it is necessary to carry out the framing adjustment for a newly replaced test pattern before proceeding to other adjustments, since the new test pattern is introduced into the optical path through the replaced image pickup lens. Thus, the alignment procedure of this is time consuming and cumbersome.
The arrangement (2) is not subject optically to the image pickup lens. Instead it requires many optical components including the bias light supply device made movable yet keeping them in precision. In detail, space between the rear portion of the image pickup lens and the front portion of the three-color separation prism system is considerably limited in the practical camera. In this limited space, there are arranged many components such as filters and the bias light supply device. In fact, the space between two adjacent components is approximately 0.5 m/m. This means that it is extremely difficult to insert another component into that space. Therefore, the arrangement (2) described above is so designed as to move out the bias light supply device and instead insert the pattern projector therein. In this case, the pattern projector must be formed so as to have the thickness of less than approximately 4.0 m/m, because the thickness of the bias light supply device itself is approximately 4.0 m/m. As a result, the arrangement (2) requires high precision, and is a complex and expensive system. In addition, it is necessary to provide the camera with additional space to install a mechanism for moving the bias light supply device away and instead inserting the pattern projector. It is therefore difficult to make the camera compact.